feat(tvix/eval): Add Compiler::thunk method for emitting thunks

The logic in this method is *very* similar to `compile_lambda`. It is
intended to be called around any expression that should be
thunked (such as function applications, attribute set values, etc.).

Change-Id: Idfbb2daa9f4b735095378fb9c39a2fd07c8cff91
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6344
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/vm.rs

@@ -435,6 +435,8 @@ impl VM {
                     }
                 }
 
+                OpCode::OpThunk(_idx) => todo!("runtime thunk construction"),
+
                 OpCode::OpFinalise(StackIdx(idx)) => {
                     match &self.stack[self.frame().stack_offset + idx] {
                         Value::Closure(closure) => closure